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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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THE EARTHQUAKE ADVANCE THEORY         185
that with the development of viscosity the rate of possible ice flow is increased. The maximum thickness of glaciers is rather small for great increase of viscosity by overloading the upper layers, but the pressure inferred would be not only direct vertical pressure of upper layers on lower, but also hydrostatic pressure of heavily-loaded upper portions of the glacier stream on the parts of the ice farther down-stream and down grade from the portion overloaded by avalanches. In narrow glaciers these avalanches doubtless went clear across the ice tongue, and where they did not do so, the same marginal area was overloaded. The final assumption is that the addition of snow to the upper glacier by the effect of earthquake shaking started a wave of advance which by applying pressure to the ice in front induced such a degree of viscosity that flowage followed, and that continued and was transmitted down the glacier until the effects of the advance wore out either because the end of the glacier was reached or because of dissipation in an expanded piedmont bulb.
By the glacier flood hypothesis it is not conceived that there was actual transfer of ice from the glacier head to the glacier end within the brief period occupied by the advance, but that there was, nevertheless, actual viscous flowage of the lower ice throughout practically the entire broken area, the state of increased flowage being progressively extended down the glacier until the effects of the thrust died out. , That is to say, there was a wave-like progression of a thrust down the glacier which, when applied, temporarily increased the viscosity of the lower ice layers, and this was followed by a rapid increase in the ice motion. At some time during the period the viscous lower ice layers of practically the entire glacier flowed forward to a greater or less distance, but no single portion of the ice advanced even a considerable part of the distance from the head to the end of the glacier. With the passage of this wave and the accompanying flowage of the ice, the rigid upper crust of the glacier, and the rigid margins, were subjected to such strains that the glacier was broken into the condition observed in the advancing glaciers. The hypothesis further conceives that there was some breaking of the margins, notably in the thinner parts of the piedmont areas, by a rigid thrust without the immediate association of flowage.
This hypothesis has been called the glacier flood hypothesis because of the resemblance to a river flood. When rain falls heavily, or snow melts rapidly, in the upper reaches of a river the effect of the sudden increase in volume extends down-stream, and if the river surface is covered with ice this is broken and piled up as the rising flood passes beneath it. The condition of the advancing Yakutat Bay glaciers is conceived by this flood hypothesis to imitate the river flood in such important ways as to warrant a comparison between the two; but there are two notable differences, first the difference in rapidity of transfer of the flood effect in the two cases, and secondly the difference in nature of the flowage. In the river the flow involves the transfer of water from the flood source to the river mouth; in the glacier there is no such transfer. There are other differences of minor character, but of scarcely enough importance for statement here.
The glacier flood hypothesis has the one merit that all the other hypotheses lack; it seems to account for all the facts. With the passage of such a wave and the associated rapidity of flowage of the under-ice it is easy to understand the accompanying breaking of the surface ice. The theory accounts equally well for the fact, observed in the advancing Lucia Glacier, that the margins of ,the glacier are more broken than